The present invention pertains to a molded filter element that includes thermally-bonded staple fibers and non-thermally bonded electrically-charged microfibers. Filter elements of this invention may be used in face masks that protect the wearer and others from exposure to contaminants.
Filter elements that use microfibers to capture airborne particulates have been known and used for many years. The microfiber-containing filter elements are commonly used in respirators to supply clean filtered air to the wearerxe2x80x94see, for example, U.S. Pat. No. 5,656,368 to Braun et al., U.S. Pat. No. 5,307,796 to Kronzer et al., and U.S. Pat. No. 4,729,371 to Krueger et al. An electric charge is typically placed on the microfibers to improve their capturing efficiency. In 1980, Kubik et al. described a method for introducing a persistent electric charge into meltblown microfibers during fiber formation (see U.S. Pat. No. 4,215,682). After the Kubik et al. development, other charging techniques were developed for making electret microfibersxe2x80x94see, for example, U.S. Pat. Nos. 4,588,537 to Klaase et al., 5,227,172 to Deeds et al., and 5,496,507 to Angadjivand et al.
When used as a filter element, webs of electrically-charged microfibers have been commonly supported by another structure. Although the webs typically possess sufficient integrity to be handleable by themselves as a mat, they do not possess sufficient structural stiffness to exhibit more than transitory shape retention. Thus, in respirators, the microfiber-containing filter webs are regularly supported by a permanently-molded shaping layer. The filter web is positioned over the shaping layer and is secured to it to assume its molded configuration. Examples of patents that disclose the use of a separate shaping layer to support a microfiber-containing filter layer in a respirator include U.S. Pat. Nos. 4,536,440 to Berg, 4,807,619 to Dyrud et al., and 4,850,347 to Skov.
In the Berg and Dyrud et al. patents, the shaping layer comprises fibers that are bonded to one another and to other fibers at points of fiber intersection throughout the molded web. The fibers that are used to achieve the desired molded configuration of the shaping layer preferably are thermally-bondable bicomponent fibers that are rather course or large in size, that is, 10 denier or larger. In Skov, the shaping layer takes the form of an open-work molded plastic mesh.
In addition to these structures, filtering webs of polymeric microfiber also have been retained in a shaped configuration through use of fabrics, spaced bands, filaments, or fibersxe2x80x94see U.S. Pat. No. 5,656,368 to Braun et al., which describes placing these shape-retaining elements across the tops of corrugations in a nonwoven web to maintain its corrugated condition. Braun et al. also explain that nonwoven webs of polymeric microfiber need to be maintained in a lofty condition in order to obtain optimal filtration performance. Filtration parameters such as pressure drop and service life can be negatively impacted when webs of polymeric microfiber are compacted.
In yet another approach to maintaining the shape of a nonwoven web of microfibers, the shape retention is provided not by an external structure such as a shaping layer but instead by the microfibers themselves. In this approach, which is described in U.S. Pat. No. 6,057,256 to Krueger et al., the microfibers are made from two components: the first being a fiber support component, and the second being a thermo-softening or bonding component. During molding, the web is heated to a temperature greater than the second component""s softening temperature to create bonds between adjacent fibers. The bonds occur where the softened components engage each other at points of fiber intersection. Thus, while the first component provides the fibrous support to the web, which prevents it from collapsing or coalescing, the second component allows it to be molded into a particular shape. The product may also include staple fibers to open or loosen the web.
With the exception of the bicomponent microfiber product described by Krueger et al., the means for supporting and providing shape to a microfiber-containing filter web is achieved by a separate or non-integral supporting structure. These products therefore require that the microfiber-containing web be manufactured separate from the supporting structure and also require that there be an apparatus for joining the two elements together to create the resulting composite article. The use of these additional manufacturing steps, and the need for the additional manufacturing equipment to accomplish these steps, adds to the final cost of the product. In addition, care needs to be taken when handling the microfiber-containing webs so that the webs do not become damaged and suffer a loss in filtration performance as taught by Braun et al.
Although Krueger et al. were able to achieve a molded microfiber-containing filter element without using a separate supporting structure, their product, however, relies on the microfiber itself for furnishing the structure to the mask. When the microfibers become bonded together for this purpose, they have a tendency to not fully serve their primary mission of filtering particles from the airstream that passes through the web.
The present invention provides, in brief summary, a new filter element that may suitably comprise or consists essentially of a porous molded web that contains thermally bonded staple fibers and non-thermally bonded electrically-charged microfibers. The molded web is retained on its molded configuration, at least in part, by bonds between the staple fibers at points of fiber intersection.
The filter element of the invention allows a shaped structure to be created without using of a separate supporting structure. The inventive filter element also does not need to bond the microfibers together for purposes of maintaining the molded shape of the nonwoven web. Because the thermally-bonded staple fibers and the non-thermally bonded electrically-charged microfibers are present in the same layer, the web can be molded into a variety of configurations without using thermally-bonded bicomponent microfibers. There also is no need for a separate supporting structure or a need to join the layer of microfibers to the additional structure.
The filter elements of the present invention can yield a desirable combination of good filtration performance and good structural integrity, even though the filter elements may be subjected to compression during the molding operation. The good performance and structural characteristics are able to be achieved despite previous thought that filtration parameters such as pressure drop and service life could be negatively impacted when a nonwoven web of polymeric microfibers was subject to additional handling such as from a molding operation. Previously known filtration structures, such as those described in the ""368 patent to Braun et al., need to maintain a lofty web condition to obtain optimal filtration performance. The present invention, however, is able to demonstrate good mechanical and filtration properties, in conjunction with a simplified operation for creating a shaped filter element. The present invention therefore can produce a molded filter element that is a good candidate for use in a molded respiratory mask.
In reference to the invention, the following terms are defined as set forth below:
xe2x80x9cdenierxe2x80x9d means the weight in grams of 9,000 meters of filament;
xe2x80x9celectrically-chargedxe2x80x9d means the fibers possess electric charge that is capable of being measured and is present on the fibers for more than transitory duration;
xe2x80x9cfilter elementxe2x80x9d means a fluid permeable structure that is capable of removing contaminants from a fluid that passes through it;
xe2x80x9cmicrofibersxe2x80x9d mean fibers that have indeterminate length and that have an average geometric fiber diameter of about 15 micrometers (xcexcm) or less;
xe2x80x9cmolded webxe2x80x9d means a structure that is substantially larger in two dimensions than in a third and that has been formed into a desired shape such as a face mask, a furnace filter, a panel or series of panels, et cetera;
xe2x80x9cmolding temperaturexe2x80x9d means the temperature at which the web is heated to accomplish molding;
xe2x80x9cnon-thermally bondedxe2x80x9d means the fibers do not substantially bond to adjacent contacting fibers after being heated to a temperature suitable for molding a web into which the non-thermally bonded fibers are contained;
xe2x80x9cporousxe2x80x9d means fluid permeable;
xe2x80x9csolidityxe2x80x9d means the percent solids in a web and is expressed as a unitless fraction where a greater number indicates a greater solids"" fraction; and
xe2x80x9csoftening temperaturexe2x80x9d means the lowest temperature at which a fiber component is softened to an extent that permits that fiber component to bond to another fiber and retain that bonded condition when cooled;
xe2x80x9cstaple fibersxe2x80x9d mean fibers that have determinate length;
xe2x80x9cthermally bondable fibersxe2x80x9d mean fibers that can bond to adjacent contacting fibers after being heated to at least above their softening temperature and being subsequently cooled;
xe2x80x9cthermally bonded fibersxe2x80x9d mean fibers that are bonded to adjacent contacting fibers after being heated to at least above their softening temperature and being subsequently cooled.